1. Field of the Invention
The present invention relates in general to an optical reading apparatus used for image input and/or output devices such as facsimile systems, and more particularly to techniques for improving reading accuracy and reproducibility of images of a subject copy.
2. Discussion of the Prior Art
In an optical reader incorporated in a conventional facsimile transmitter or other types of an image input/output device, a subject copy is read in a bi-tone method wherein each reading spot is read as either "black" or "white" spot, the black and white spots being represented by respective two levels of a binary image signal. While this bi-tone reading method provides good reproducibility of characters such as letters, the method suffers from poor reproducibility of a photograph or other images which have a series of achromatic tones or a full range of gray scale from black to white. Namely, the local reading spots on a photograph which have different or varying achromatic tones are classified into black and white spots, with the tones compared with a predetermined threshold. Consequently, the photograph reproduced according to the image signals obtained by the optical reader cannot be perceived as a photograph having a shade variation. In the light of the above drawback, an optical reader capable of reading a gray scale image such as a photograph or picture is proposed. This optical reader is adapted to read a subject copy in a dither-matrix fashion wherein each reading spot on the entire area of the subject copy is read as a dither matrix of black and white dots, for example, 4.times.4 or 3.times.3 matrix of dots, so that the achromatic tone of the relevant reading spot is expressed as one of eight or sixteen different levels. This multi-tone or dither-matrix reading method provides improved reproducibility of photographs or other gray scale images.
However, the image-bearing surface of a subject copy frequently consists of both bi-tone areas bearing characters, and gray scale areas bearing photographs or other images having a shade variation. If this image-bearing surface is read in the bi-tone reading method, the reproducibility of the gray scale areas is not satisfactory for the reason described above. If the image-bearing surface is read in the dither-matrix method, the gray scale areas can be read with good reproducibility, while the bi-tone areas are read with some shade variation or different achromatic tones in the black or image portions such as letters. Sometimes, the boundaries between the reproduced letters and the white or non-image portions are unclear or indefinite, and the small-sized letters cannot be easily perceived.
One type of the optical reader capable of reading in the dither-matrix mode indicated above uses different threshold values with which comparators compare amounts or intensities of light beams which are reflected by adjacent local segments of a reading spot on the subject copy, in order to determine the level of binary image signals representative of the achromatic tone (black or white) in the respective local segments. The binary image signals of the adjacent local segments constitute dither-matrix pattern data of the reading spot. Described more specifically, a local segment having a given degree of achromatic tone is read as a white dot if the threshold value used for the comparator is relatively low, and is read as a black dot if the threshold value is relatively high. Consequently, the use of a plurality of different threshold values for the adjacent local segments of each reading spot makes it possible to represent the achromatic tone of the reading spot in a plurality of steps with a dither matrix pattern of the black and white local segments. Thus, the image-bearing surface may be read and reproduced in a dither-matrix or multi-tone mode.
In the meantime, the assignee of the present application proposed an optical reader as disclosed in U.S. Pat. No. 4,689,692 to Harano et al. corresponding to laid-open Publication No. 61-123357 of unexamined Japanese Patent Application, wherein each light-emitting element is driven by predetermined different amounts of drive power so that the light beams produced by the light-emitting element to irradiate the adjacent local segments of a reading spot have different intensities, so that dither-matrix pattern data may be prepared for the reading spot. In this case, the comparator uses a single threshold value. However, the physical properties and operating characteristics of the individual light-emitting elements for producing light beams are different and are chronologically changed, whereby the amounts or intensities of the light beams produced by the individual light-emitting elements may be different or inconsistent and are varied during the service life. Similarly, the output values of the individual light-sensitive elements connected to the comparators are not consistent and are chronologically varied. Therefore, the obtained dither-matrix pattern data or the image reproduced according to the dither-matrix pattern data suffers from insufficient reproducibility due to influences by the differences, variations and chronological changes of the optical elements.
To eliminate the adverse influence of the differences, variations and chronological changes of the optical elements on the reading accuracy of the optical reader, it has been proposed to use a suitable reflector member which has a reference reflector surface having a reference light reflectance value. By using the reflector member, the optimum amount of a light beam produced by each light-emitting element can be determined prior to an image reading operation, by adjusting the drive power applied to the light-emitting element so that the output value of a comparator producing an image signal reaches an optimum value when the reference reflector surface is irradiated by the light beam.
For example, a sheet holder member which cooperates with a reading surface of a reading head to hold a sheet of a subject copy is utilized as a reflector member having the reference reflector surface. The sheet holder member is supported movably toward and away from the reading surface of the reading head. The sheet holder member has a recessed portion formed in the surface facing the reading head. The bottom surface of the recessed portion serves as the reference reflector surface used for determining the optimum amount of the light beam produced by each light-emitting element. The sheet holder member is movable between its advanced position in which the reflector surface is irradiated upon determination of the optimum amount of the light beam, and its retracted position in which the sheet holder member functions to guide the sheet of a subject copy in cooperation with the reading head.
The reflector surface is provided on the bottom of the recessed portion, in order to prevent the reflector surface from being soiled by contact with the image-bearing surface of the subject copy. While this arrangement assures the maintenance of the predetermined light reflectance value of the reflector surface, the same arrangement results in a drawback. Described in more detail, the distance between the reflector surface and the reading surface of the reading head is different from the distance between the image-bearing surface of the subject copy and the reading surface of the reading head. This difference causes a difference between the optimum amount of a light beam (optimum amount of drive power to drive each light-emitting element) determined with respect to the reflector surface, and the optimum amount of the light beam which is actually used to irradiate the image-bearing surface of the copy sheet. In other words, the optimum amount determined by irradiating the reflector surface is not optimum for the actual reading of the copy sheet.